In recent years, as having a high operation voltage and high energy density, non-aqueous electrolyte secondary batteries, especially lithium-ion secondary batteries, have come into practical use as power sources for driving portable electronic instruments such as cell phones, laptop computers, video camcoders, and have made rapid progress. Lithium-ion secondary batteries are becoming the mainstream of small-sized secondary batteries and the production volume thereof is on the increase.
Lithium-ion secondary batteries are not only for small-sized customer applications, but the technical development thereof into large-sized batteries having a large capacity for power storage, electric vehicles and the like has been accelerated, and in particular, lithium-ion secondary batteries for hybrid electric vehicles (HEVs) are under rapid development. Furthermore, in the area of power-sources for driving electric tools and the like, required to have very high output power, high-output type lithium-ion secondary batteries as replacements of conventional nickel-cadmium batteries and nickel-metal hydride batteries are under rapid development.
Herein, the aforesaid high-output type lithium-ion secondary batteries largely differ in applications and required performance from lithium-ion secondary batteries for small-sized customer applications. In the case of lithium-ion secondary batteries for HEVs, for example, an engine of an HEV needs power-assisting and regenerating within a fraction of a second with a limited capacity of the battery, necessitating considerably high input/output power of the battery. It is therefore necessary to give preference to a high input/output characteristic over the other battery characteristics, and to make the internal resistance of the battery as small as possible.
Consequently, in addition to development and selection of active materials and electrolytes, there have been attempted reconsideration of current collecting structures of electrodes, reduction in resistance of battery constituents, an increase in electrode reaction area by making the electrode thinner and longer, and the like.
As for positive electrode active materials of the lithium-ion secondary batteries for HEVs, LiNiMO2 type active materials have been considered as most suitable—and the development thereof has been advanced (e.g. Japanese Laid-Open Patent Publication No. Hei 5-242891, Japanese Laid-Open Patent Publication No. Hei 9-231973, Japanese Laid-Open Patent Publication No. Hei 9-293497, and Japanese Laid-Open Patent Publication No. Hei 9-237631). There have further been conducted studies on production methods of the LiNiMO2 type active materials (e.g. Japanese Laid-Open Patent Publication No. Hei 10-27611, Japanese Laid-Open Patent Publication No. Hei 11-60244, and Japanese Laid-Open Patent Publication No. Hei 11-219706).
As thus described, the lithium-ion secondary battery required to have high input/output power needs to sustain large current pulse charge or discharge from about 50% state of charge. It is thereby necessary to make the internal resistance of the battery as small as possible.
The internal resistance of the battery here is comprised of: a resistance element due to battery constituents, an electrolyte and the like; and a resistance element due to a battery reaction. The internal resistance of the battery occurring within the temperature range of a normal temperature to a high temperature can be reduced by reducing the former resistance element, thereby enabling the battery to have high input/output power. In the case of the internal resistance of the battery occurring within the low temperature range of 0° C. and below, however, the latter resistance element due to a battery reaction contributes far more than the former resistance element, making it difficult for the battery to have high input/output power without reduction in the latter resistance element.
Some of the aforesaid conventional LiNiMO2 type active materials are produced by mixing nickel hydroxide, a compound containing the element M to be incorporated in the nickel hydroxide, and a lithium compound such as lithium hydroxide, and heating the mixture. Further, some of the aforesaid LiNiMO2 type active materials are produced by preparing NiM(OH)2, incorporated with M by a coprecipitation method, mixing prepared NiM(OH)2 with a lithium compound, and then heating the mixture. The LiNiMO2 active materials thus obtained are somewhat different in performance, depending on the kind of M, composition and the like, but not largely different in physical properties, and it is therefore difficult to suppress the aforesaid increase in resistance due to a battery reaction in a low temperature environment. For example, when conductivity of an electrolyte excessively decreases at a low temperature, the ability of the active material to absorb and desorb lithium significantly deteriorates, leading to an unsatisfactory high input/output characteristic of the battery.